Harmonic edge synthesizer, enhancer and methods

ABSTRACT

An edge enhancer for enhancing edges within a video image, forms an edge enhancement signal of odd harmonics of a sinusoid representative of the edge content in an incoming signal. The edge enhancement signal may be added to a version of the incoming signal, thereby sharpening the edge in the incoming signal. This allows edges to be enhanced, increasing the image&#39;s rise-time without adding overshoot. Sharper edges are thus produced in much the same way as a square wave may be formed of odd harmonics of a sinusoid.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefits from U.S. provisional application Ser.No. 60/755,083 filed Jan. 3, 2006, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to image processing, and moreparticularly to enhancing edges in images. The invention is particularlywell suited in a video image scaler.

BACKGROUND OF THE INVENTION

Today, television is in the midst of a transformation. Standardtelevision receivers capable of receiving and presenting signalsbroadcast since the 1940s are being replaced by televisions capable ofpresenting higher definition signals (so called high definition (HD)signals). HD signals are typically broadcast as digital signals, andcurrently result in images having resolutions of up to 1920×1080 pixels.This is a vast improvement over conventional standard definition (SD)signals that typically result in images having between 480 and 525lines, each equivalent to between about 640 to 720 pixels. HD signalsthus allow for images having greater image detail and sharpness,allowing for an enhanced viewing experience. As an added benefit,television images may be presented on larger and larger displays.

As many existing television broadcasts are still in SD format, and asmany existing programs have been stored and recorded in SD format, newerHD televisions are typically capable of presenting both HD and SDimages. SD images are simply enlarged or scaled to fit available spaceon an HD display.

However, any time an SD image is scaled to HD, a relatively low contentsignal is converted for presentation into a display format that has morepixels and allows for more rapid image transitions. The scaled SD imageis thus far less sharp than a true HD image.

To address deficiencies in scaled SD signals, edge enhancers are oftenused to enhance edges within the up-scaled image. Good edge enhancementis difficult to achieve, because it simultaneously requires activelyspeeding up slower edges without causing ringing. Ringing arises becauseedge-enhancing adds high-frequency components to signals which tend toring when scaled.

Known edge enhancers add the incoming signal to a differentiated,amplified, and clipped version of this signal. Such enhancers wereinitially designed to enhance VCR images with frequency responses wellbelow that of broadcast television. Early edge enhancers operated onlyon the colour (or UV) portion of the signal. The techniques have sincebeen extended to the luminance portion of the signal.

Differentiation, however, inherently emphasizes high frequency noise.Thus, these conventional edge enhancers basically enhance the lowfrequencies that are already there—they do not really modify the edgeand add new high frequencies, except possibly by non-linear clipping.This leads to the “edge around the object” type of sharpness enhancementthat is commonly observed on existing TVs.

Further, these edge enhancers often require a circuit which mustexplicitly decide when an edge is present using a logic circuit. Suchdecision circuits often fail on complicated source images. Methods whichalter the time scale to move the regions before or after an edge aretypical of those which require this type of decision. Further, thedecision circuits are confused by thin or multiple or close edges. Toalleviate this problem, complex and often ineffective circuits may alterhow edges are processed in regions of close multiple edges.

Accordingly, there is a need for new edge enhancement techniques andcircuits.

SUMMARY OF THE INVENTION

Exemplary of the present invention, an edge enhancer forms an edgeenhancement signal of odd harmonics of a sinusoid representative of theedge content in the incoming signal. The edge enhancement signal may beadded to a version of the incoming signal, thereby sharpening the edgein the incoming signal. This allows edges to be enhanced, increasing theimage's rise-time without adding overshoot. Sharper edges are thusproduced in much the same way as a square wave may be formed of oddharmonics of a sinusoid.

The edge enhancer may be combined with an image scaler. A scaled versionof the edge enhancement signal including harmonics of the originalsignal may then be added to a scaled version of the signal. The image tobe scaled may be analysed prior to scaling, to calculate an edgeenhancement signal based on the frequency content of the image.Conveniently, the edge enhancement signal has relatively low frequencycontent, and thus may be easily scaled. The scaled edge enhancementsignal may then be combined with the scaled version of the originalsignal.

In accordance with an aspect of the present invention, there is provideda method, comprising: analysing a plurality of adjacent pixels in apixel stream to determine a sinusoid approximating an edge in theplurality of adjacent pixels; generating an edge enhancement signalincluding a sum of odd harmonics of the sinusoid.

In accordance with another aspect of the present invention, there isprovided a method comprising, storing values representative of aplurality of adjacent pixels in an image; analysing the plurality ofadjacent pixels to determine a sinusoid representative of an edge in theplurality of adjacent pixels; generating an edge enhancement signalincluding a sum of odd harmonics of the sinusoid; combining arepresentation of at least some of the values and the edge enhancementsignal, to form an edge enhanced signal.

In accordance with another aspect of the present invention, there isprovided a method comprising, buffering values representative of aplurality of adjacent pixels in an image; analysing the plurality ofadjacent pixels; combining at least of some of the values of theplurality of adjacent pixels to form a scaled representation of theplurality of adjacent pixels; generating an edge enhancement signalbased on the analysing; combining the scaled representation of theplurality of adjacent pixels and the edge enhancement signal, to form anedge enhanced, scaled signal.

In accordance with yet another aspect of the present invention, there isprovided an edge enhancement circuit comprising, a buffer for storing aplurality of adjacent pixels; a filter for determining a sinusoidrepresentative of an edge in the plurality of adjacent pixels; a signalgenerator for generating an edge enhancement signal including a sum ofodd harmonics of the sinusoid.

In accordance with yet another aspect of the present invention, there isprovided a method comprising, buffering values representative of aplurality of adjacent pixels in an image; analysing the plurality ofadjacent pixels; combining at least of some of the values of theplurality of adjacent pixels to form a scaled representation of theplurality of adjacent pixels; generating an edge enhancement signalbased on the analysing; combining the scaled representation of theplurality of adjacent pixels and the edge enhancement signal, to form anedge enhanced, scaled signal.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate by way of example only, embodiments ofthe present invention,

FIG. 1 is a simplified block diagram of a scaler and edge enhancer,exemplary of an embodiment of the present invention;

FIG. 2 is a block diagram of an edge synthesizer of scaler and edgeenhancer FIG. 1;

FIGS. 3A-3D depict the addition of odd harmonics to enhance an edge;

FIG. 4 is further block diagram of edge synthesizer of the scaler andedge enhancer of FIG. 1; and

FIGS. 5A-5D are block diagrams of various component of the edgesynthesizer FIG. 4.

DETAILED DESCRIPTION

FIG. 1 is a simplified schematic block diagram of an edgeenhancer/scaler 10, exemplary of an embodiment of the present invention.Edge enhancer/scaler 10 may be formed as a portion of an integratedcircuit, using conventional integrated circuit design and fabricationtechniques. Enhancer/scaler 10 is particularly well suited for use in avideo processor, in digital television receiver, a television adapterfor a computer, a video decoder, a set-top television or cable receiver,a media player or the like. The video processor may be suitable for usein a computer, television, flat panel monitor, media player (includingDVD, PVR or the like), in a camera, or other device requiring thedisplay of digitized images.

As illustrated, edge enhancer/scaler 10 includes a buffer 12 suitablefor storing a number of adjacent pixels within a stream of pixels y(n).The pixel values may be values of pixels in RGB colour space, YUV colourspace, or a colour or luminance component of the pixels. A multi-tapscaler 14 combines adjacent pixels to form a scaled version, Y(n), of animage represented by the pixel stream y(n). Scaler 14 may be aconventional multi-tap scaler, or may be an adaptive scaler, as forexample disclosed in a co-pending application entitled “IMAGE ANALYZERAND ADAPTIVE IMAGE SCALING CIRCUIT AND METHODS”, naming Edward Callwayas inventor, and filed concurrently herewith.

A multi tap scaler generates scaled pixels by forming a weighted sum ofadjacent pixels in the image. In the depicted embodiment, an m tapscaler is used. That is a scaled pixel Y(j) is formed of the weightedsum of m adjacent pixels,

${Y(j)} = {{1/m}{\sum\limits_{i = 0}^{m - 1}{{Si}*{y( {i + j} )}}}}$

where S_(i) are scaling coefficients.

For each m pixels received in buffer 12, muti-tap scaler 14 generatesone or more output pixels. If scaler 14 is to up-scale an image, scaler14 outputs M>N pixels, for each N pixels entering buffer 12. Asrequired, the m scaling coefficients S_(i) may be updated for eachoutput pixel. Suitable sets of scaling coefficients may be stored inmemory (not shown).

Similarly, if scaler 14 downscales an image, scaler 14 outputs M<Npixels, for each N pixels entering buffer 12. Again, the m scalingcoefficients S_(i) may be updated for each output pixel.

Scaler 14 may thus be clocked at a rate proportional to (i.e. M:N timesthe rate) the arriving pixel stream.

Enhancer/scaler 10 can thus be used to scale a two dimensional imagevertically or horizontally, by providing a pixel stream arranged incolumns or rows to enhancer/scaler 10. If an image is to be scaled bothvertically and horizontally, two separate enhancers/scalers may be used.

An edge synthesizer 16 analyzes the incoming pixels to determine theirfrequency content, and generate an edge enhancement signal. As willbecome apparent, synthesizer 16, analyses the incoming stream m pixelsat a time, to form a harmonic enhancement function that in turn is usedto produce an enhancement value that may be used to enhance an edge inthe scaled version of the pixels. The edge enhancement signal is formedof odd harmonics of the frequency content of an edge in the group ofadjacent pixels. Although m pixels are used by synthesizer 16, thenumber of pixels used to analyse and form the edge enhancement signalmay be different than the number of pixels that are combined by scaler14. That is, scaler 14 could use m₁ taps, while synthesizer 16 couldanalyse m₂ pixels.

Synthesizer 16 operates on the premise that an edge within the signalmay be modelled as a sinusoid, and that sharp edges can be generated asthe sum of odd harmonics of that sinusoid. As will be apparent, as usedherein, a sum of harmonics refers to a weighted sum of the harmonics.Thus, exemplary synthesizer 16 forms a sinusoid representative of theedge of interest. As will become apparent, synthesizer 16 forms asinusoid, by band-pass filtering the signal including the edge ofinterest. The band-pass filter has a sinusoidal impulse response. Inthis way, synthesizer 16 creates a sinusoid having a frequency equal tothe frequency of the sinusoid modelling the edge of interest. In thedepicted embodiment, synthesizer 16 may then use the idea of Fouriersquare wave synthesis as a basis for forming an edge enhancement signal.However, different amplitudes of the harmonics may be used to produce asharper edge with only two or three odd harmonics.

Once the edge enhancement signal has been generated, adder 18 adds thegenerated edge enhancement signal to the scaled output of scaler 14.Specifically, adder 18 adds the value of the generated edge enhancementsignal to pixels in the scaled image.

In the depicted embodiment, the following trigonometric observations arecombined to generate the harmonic enhancement value:Sin(3x)=sin(x)*(−1+4*cos²(x))Sin(5x)=sin(x)*(1−12*cos²(x)+16*cos⁴(x))Sin(7x)=sin(x)*(−1+24*cos²(x)−80*cos⁴(x)+64*cos⁶(x))H ²=(H sin)²(x)+(H cos)²(x)

Assuming that an edge of interest in the m buffered pixels in the streamy(n) may be approximated by a sinusoid of frequency ω/2π, an edgeenhancement signal may take the form,f ₁(n)=G*((A−1)*sin(ωn)+B*sin(3ωn)+C*sin(5ωn)+D*sin(7ωn))f ₂(n)=G*sin(ωn)*(K+L*cos²(ωn)+N*cos⁴(ωn)+Q*cos⁶(ωn)).Conveniently, f₂(n) is a trigonemtric equivalent (or approximation) off₁(n) using only functions of ω and not functions of harmonics of ω(i.e. 2ω, 3ω, etc.).

Further, sin(ωn) and cos(ωn) representative of the edge, may beapproximated from the m pixels in the pixel stream y(n) by band-passfiltering the adjacent pixels in the stream. That is,H sin(ωn)≈F(y(n))=a band-pass filtered version of the source y(n) withno phase shift, andH cos(ωn)≈F(y(n))=a band-pass filtered version of the source y(n) with a90° shiftBy choosing the nature of the band-pass filters, and more specificallyits center frequency, and pass-band, an appropriate filter may detectedges having a desired slope, approximated by sinusoids in thisfrequency range.

Once the edge has been approximated as a sinusoid, synthesizer 16derives an edge enhancement function f₃ from m pixels in the incomingpixel stream y(n), as depicted in FIG. 2:f ₃(n)=G*H sin(ωn)*(K+L*((H cos)²(ωn)/H ²)+N*((H cos)²(ωn)/H ²)² +Q*((Hcos)²(ωn)/H ²)³)Values K, L, N, and Q may be stored in programmable registers.

For illustration, the values of a series of m pixels y(n) having a slowedge, are depicted in FIG. 3A, and may be approximated as a sinusoid(e.g. sine wave) depicted in FIG. 3B. Odd harmonics of the sinusoid maythen be calculated as in FIG. 3C. Without scaling, the weighted sum ofodd harmonics and the original signal y(n) results in an edge enhancedsignal, as depicted in FIG. 3D.

A further simplified block diagram of edge synthesizer 16 is depicted inFIG. 4.

Synthesizer 16 re-calculates the parameters of the enhancement functionf₃(n) for each new pixel in buffer 12, using m adjacent pixels in buffer12.

The trigonometric identities above use sine and cosine terms with anamplitude of one (1). For most manipulations, amplitude is not aconcern. Of course, a real signal typically has a non-unity amplitude.So, equation f₂(x) is only an approximation for a real video signal.

Viewed as a Fourier series, the source y(n) is made up of a sum of manysignals of differing frequencies, amplitudes, and phases. Narrowbandpass filters 20, 22 may be used to produce two narrow bandquadrature signals that may be viewed as H sin(ωn) and H cos(ωn). [Infact, for a time varying signal, H is really H(t), a varying signal].

Specifically, as illustrated, pixel values stored in buffer 12 areprovided to band-pass sine filter 20, and band-pass cosine filter 22.Filters 20 and 22 are band-pass filters, having appropriately chosencenter and pass-band frequencies. They may be formed as FIR filters.Further, filters 20 and 22 have sine and cosine impulse response. Assuch, filters 20, 22 perform low frequency spectral analysis of the mpixels in buffer 12 and act as quadrature band-pass filters and onlypass those components of the signal y(n) with frequencies at 0-50% ofthe sampling frequency of the pixels (i.e. the Nyquist frequency).Filters 20, 22 further generate a sine and cosine function,respectively, having a frequency corresponding to the frequency of the mpixels of pixel stream y(n). A suitable sine filter 20 and cosine filter22 will be apparent to those of ordinary skill. For example, arepresentative sine filter could be formed as a FIR filter havingcoefficients {−0.25, 0, 0.5, 0, −0.25}. A representative cosine filtercould be formed as a FIR filter having coefficients {0, 0.5, 0, −0.5,0}.

Alternative band-pass filters useable as filters 20 and 22 are detailedin Richard G. Lyons, Understanding Digital Signal Processing (PrenticeHall, 2ed, 2004).

Synthesizer 16 thus formsf ₃(n)=G*H sin(ωn)*(K+L*((H cos)²(ωn)/H ²)+N*((H cos)²(ωn)/H ²)² +Q*((Hcos)²(ωn)/H ²)³)

H sin(ωn) term is easily obtained, as it appears naturally as the sourcesignal y(n) is filtered using filter 20. Deriving cos² is slightly lessstraight forward. H cannot easily be separated from H cos, as inequation f₂(n). However, cos²(ωn) may be derived from (H cos)²(ωn)divided by H². H² is easily obtained as, H²=(H sin)²(ωn)+(H cos)²(wn).As (H cos)²(ωn) is already required, so only (H sin)²(ωn) has to begenerated with a multiply. Conveniently, H does not to be explicitlycalculated—H² may be calculated for the cosine terms.

As a result, one divide operation is used is used in synthesizer 16. Ofcourse, the minimum value of H² should be limited so that very smallnoise signals do not get amplified into false edges.

The term G controls the gain of the enhancement signal—when G=0 noenhancement signal is output by synthesizer 16. B, C, D in f₁ controlthe weight of the harmonics, and may be programmable to allow finetuning the shape of the enhancement signal. A may be viewed as simply aboost/cut control at the centre frequency of the filter.

Edge synthesizer 16 controls the values A, B, C, and D by way of K, L, Nand Q stored values stored in programmable registers. The actualregister values K, L, N and Q are easily derived from A, B, C, D, asfollows:

$\begin{matrix}{{f(x)} = {G*( {{( {A - 1} )*{\sin(x)}} + {B*( {{\sin(x)}*( {{- 1} + {4*{\cos^{2}(x)}}} )} )} +} }} \\{{C*( {{\sin(x)}*( {1 - {12*{\cos^{2}(x)}} + {16*{\cos^{4}(x)}}} )} )} + {D*( {{\sin(x)}*} }} \\  ( {{- 1} + {24*{\cos^{2}(x)}} - {80*{\cos^{4}(x)}} + {64*{\cos^{6}(x)}}} ) ) ) \\{= {G*( {{( {A - 1} )*{\sin(x)}} - {B*{\sin(x)}} + {B*{\sin(x)}*4*\cos^{2}(x)} +} }} \\{{C*\sin(x)} - {C*{\sin(x)}*12*{\cos^{2}(x)}} + {C*{\sin(x)}*16*}} \\{{\cos^{4}(x)} - {D*{\sin(x)}} + {D*{\sin(x)}*24*\cos^{2}(x)} -} \\ {{D*\sin(x)*80*{\cos^{4}(x)}} + {D*{\sin(x)}*64*{\cos^{6}(x)}}} ) \\{= {G*( {{( {( {A - 1} ) - B + C - D} )*{\sin(x)}} +} }} \\{( {B\; - {3C} + {6D}} )*{\sin(x)}*} \\{{4*{\cos^{2}(x)}} + {( {{4C} - {20D}} )*{\sin(x)}*4*\cos^{4}(x)} +} \\ {( {16D} )*{\sin(x)}*4*{\cos^{6}(x)}} ) \\{= {G*{\sin(x)}*( {( {( {A - 1} ) - B + C - D} ) + {( {{4B} - {12C} + {24D}} )*}} }} \\ {{\cos^{2}(x)} + {( {{16C} - {80D}} )*{\cos^{4}(x)}} + {( {64D} )*{\cos^{6}(x)}}} )\end{matrix}$ Thus, K = ((A − 1) − B + C − D) L = (4B − 12C + 24D)N = (16C − 80D) Q = 64D

Programmable registers (not shown) storing values K, L, N and Q may formpart of enhancer/scaler 10 and be programmed under software control.Values K, L, N and Q qualitatively control the edgeenhancement/synthesis of enhancer/scaler 10. They typically are notreprogrammed, and remain constant for sequential sets of adjacentsixteen pixels.

As noted, edge synthesizer 16 generates one edge synthesis functionf₃(n) for every m pixels in y(n), and one enhancement value for these mpixels. In order to align the generated edge enhancement value with them incoming pixels (or scaled versions thereof), the output of edgesynthesizer 16 may need to be delayed to generate f₃(n) for any madjacent pixels. In the depicted embodiment, an edge enhancement valueis most representative at the center of the plurality of pixels inbuffer 12. Thus, for a 15 tap cosine filter, edge synthesizer 16 maygenerate an edge enhancement value, best used to enhance the 8^(th)pixel in buffer 12.

Five pixels about the center pixel in buffer 12 are also passed frombuffer 12 through a high pass filter 24. High pass filter 24 performs ahigh frequency spectral analysis of the block of m pixels in buffer 12,by detecting the portion of the buffered pixel signal that is highfrequency. The greater the proportion of the block of m pixels in buffer12 that has a high frequency (e.g. between 75-100% of Nyquist), the lessedge enhancement the lower frequency part of the block will besubjected. A suitable high pass filter 24 may have transfer functionapparent to those of ordinary skill, and may for example be formed as athree-tap FIR filter having coefficients {−0.5, 1, −0.5}.

Conventional squaring blocks 26, 28, multipliers 34, 36, summer 30 anddivider 32 are arranged to combine the outputs H sin(n) and H cos(n) oflow pass filter 20 and 22, to produce H sin(n)*(K+L*cos²(n)+N*cos⁴(n)+Qcos⁶(n)) to produce signal generation block 40, as illustrated in FIG.4.

The output of high pass filter 24, G, controls the gain of the edgeenhancement that takes place with the lower frequency part of thesignal. G is included in the equation for f₃(n) above. G thus controlsthe intensity of the edge enhancement and limits edge enhancement if thesignal has significant high frequency components.

FIG. 4 further illustrates where relatively simple scalers “B” 52 a (or52 b), 50 a and 50 b may be included as part of synthesizer 16.Conveniently, outputs of the filters 20, 22, and envelope detector 42(the sin and cos filtered signals, and the envelope of the highfrequency amplitude signal HFAmp, or the gain signal G) are all low passsignals and can be scaled with lower order (i.e. fewer tap) and cheaperfilters than the main pixel stream y(n). In the depicted embodiment,scalers 52 a (or 52 b) and scalers 50 a and 50 b can take the form ofbilinear scalers, and may include suitable delay blocks for storing twosequential outputs of envelope detector 42 and filters 20 and 22. Ofcourse, scalers 50 a, 50 b and 52 a (or 52 b) may be higher orderscalers. Similarly, scaling coefficients may be stored in memory (notshown), and the bilinear scalers 52 a (52 b), 50 a and 50 b may beclocked at a rate consistent with scaler 14 (i.e. at a rate proportionalto M:N the rate of arriving pixel stream). The resulting scaled signalat the output of scalers 52 a (52 b), 50 a and 50 b is scaled to a sizeconsistent with the scaled signal Y(n), but may still have frequencycontent less than the Nyquist sampling frequency of the original pixelstream. Moreover, even if the main signal y(n) can only adequately bescaled with a ten tap or higher filter, the analysis signals may bescaled with as little as a two tap bilinear filter.

As edge enhancement synthesizer 16 only processes low frequency portionsof the signal y(n), two parallel processing paths are used—one(including filters 20 and 22) for low frequency parts of the signal thatrequire edge enhancement, and one (including filter 24) for the rest ofthe signal (the higher frequency parts) for which edges should be leftalone and not sped up. Thus, the signals generated by filters 20, 22 andenvelope detector 42 are band-pass or low pass filtered analysissignals. The band-pass/low-pass filtered analysis signals are thenscaled by scalers 50 a, 50 b and 52 a and 52 b.

Unlike conventional edge enhancers, edge enhancer 16 does not include adifferentiator, edge detector or threshold detector. Instead threeenhancement/analysis signals are derived from the input stream: H sin, Hcos, and G. By band-pass filtering the input stream, only slow edgeswith a slope of interest within the pass-band of the band-pass filters20, 22 are enhanced. The envelope of the high pass filter 24 controlsthe gain.

Buffer 12 may be formed as a conventional m storage location shiftregister, as depicted in FIG. 5A. Incoming pixels are stored asdepicted. As the next pixel arrives, pixels are shifted right, and theoldest pixel is shifted out of buffer 12. As noted, the current m pixelsin buffer 12 are analysed and serve as a basis for calculating thefiltered values H sin, and H cos.

The H sin term may be produced with a conventional band-pass FIR of thetype [ . . . a, b, c, b, a . . . ]. That is the FIR may be mirrored, oddlength, with no phase shift, as depicted in FIG. 5B. The H cos term maybe produced with a conventional multi-tap band-pass FIR of the type [ .. . d, e, 0, −e, −d . . . ] i.e. mirrored, odd length, broadband 900phase shift, as depicted in FIG. 5C. Suitable FIR filters are detailedin Understanding Digital Signal Processing, supra.

The frequency response of the H sin and H cos should be well matched inthe pass-band. For standard definition video signals sampled at 13.5Mhz, they may be derived by noting that edges with frequencies in the0.5 to 2 MHz region should be boosted. Those above and below need not beenhanced. Optionally, filter coefficients may be programmable, so thatthese can be tuned and other edge rates may be enhanced. Similarly, withother coefficients, other edge rates could be enhanced for other signaltypes.

Conveniently, sin and cos filters 20, and 22 are relatively short whilestill meeting the basic frequency response goals. This limits the edgeenhancement from spreading past the edge.

A delay block used in scalers 52 a and 52 b is further illustrated ineach of FIGS. 5B and 5C.

An example high-pass filter 24 is schematically depicted in FIG. 5D. Asillustrated, high pass filter 24 and envelope detector 42 includes afive tap filter that is provided five pixels within a block in buffer12. Again, to align the value of G with the input stream, five pixels ofinterest about the center pixel within buffer 12 are used for analysis.Four cascaded delay elements generate delayed versions of the signal,and the largest is chosen, in order dampen response of filter 24.

Once f₃ is formed, it may then be added by adder 18 to the scaledversion of pixels in buffer 12, as produced by scaler 14. This resultsin sped up edges at around the frequencies of the Fourier harmonics.

For vertical scaling, filters 20 and 22 may be shorter (e.g. only 5 tapslong), as fewer lines are typically available. The boost is shifted upto midband as a result. This is not unreasonable because pixels in acolumn have few natural low pass components, unlike pixels in a line.

Harmonics used by synthesizer 16 are easily extensible: To drop the7^(th) harmonic, D=0, which will remove the Q and cos⁶(x) termsentirely. To add the 9^(th) harmonic, synthesizer 16 could be modifiedto add an R*cos⁸(x) term. A person of ordinary skill will readilyappreciate how to otherwise extend or modify the function f₃ used insynthesizer 16.

If no scaling is required, as for example in an analog CRT TV, an edgeenhancer like synthesizer 16 may be placed anywhere in the signal pathwith adequate bandwidth to handle the increased frequency content.

As should now be appreciated, placement of an edge enhancer in a digitalvideo system creates numerous challenges.

Specifically, if edge enhancement is performed prior to scaling animage, harmonics of an input signal near and past the Nyquist frequencylimit for the signal must be determined. The result is that thefrequency of the enhancement signal entering the scalar may be placedoutside of the Nyquist limit. This may cause artifacts, such asaliasing, ringing, or loss of edge enhancement or definition afterpassing through a video scaler.

If the edge enhancement is performed after scaling the image, the signalused for enhancement might only be at 15-20% of the Nyquist frequency.Since edge enhancing involves looking at the lower frequency componentsof a given signal, only 5% of the near Nyquist signal may be available.Analysing a signal at only 5% of Nyquist requires significant samplingand therefore a long and costly filter. In addition, as the scalingratio changes frequently depending on the source content or user zoomrequirements, the filter coefficients would have to be recalculated inreal time, or many filters would have to stored in advance. This is alsocostly.

Conveniently synthesizer 16 allows edge enhancement using odd harmonicswhile performing the analysis on the original signal prior to scaling,without generating signals outside the Nyquist limit. This isfacilitated by analysing the original signal y(n), without edgeenhancing, to form an analysis signal having at least one low pass (orband pass) signal. The analysis signal may then be scaled and used tosynthesize high frequency components used to form an edge enhancementsignal. The original signal and the scaled, edge enhancement signal maybe combined. That is, the edge enhancer/scaler 10 analyzes the signalprior to scaling (i.e., while the relevant slow signal y(n) is typicallyat <30% of Nyquist, which lets us work with a much shorter filter). Oncethe signal has been analyzed and calculations are made to yield signalcomponents that result in sharper edges, using a scaled edge enhancementsignal that is scaled prior to the addition of those edge sharpeningcomponents.

As should now be apparent, although enhancer/scaler 10 has been depictedas a hardware circuit, an equivalent enhancer/scaler could easily beformed in software, firmware, or the like.

In an alternate embodiment, an enhancer/scaler like enhancer/scaler 10could be formed using multiple edge enhancement synthesizers, likesynthesizer 16, but each having filters tuned to edges of differentfrequencies. Each such synthesizer may then independently, likesynthesizer 16. The output of the multiple synthesizer may then besummed with the output of a multi-tap scaler, like scaler 14, in orderto form an edge enhanced signal having edges of multiple slopesenhanced.

Of course, the above described embodiments are intended to beillustrative only and in no way limiting. The described embodiments ofcarrying out the invention are susceptible to many modifications ofform, arrangement of parts, details and order of operation. Theinvention, rather, is intended to encompass all such modification withinits scope, as defined by the claims.

1. A method, comprising: first generating a scaled output using aplurality of adjacent pixels of a video image, said first generatingperformed by one or more circuits; analysing said plurality of adjacentpixels in a pixel stream to determine a sinusoid approximating an edgein said plurality of adjacent pixels by said one or more circuits;second generating an edge enhancement signal by said one or morecircuits based on said analyzing, said edge enhancement signal includinga sum of odd harmonics of said sinusoid; and adding said scaled outputand said edge enhancement signal by said one or more circuits.
 2. Themethod of claim 1, wherein said analysing comprises band-pass filteringsaid plurality of adjacent pixels to form at least one band-passfiltered analysis signal.
 3. The method of claim 2, wherein saidband-pass filtering forms a sinusoid of the form H sin(ωn) having afrequency approximating said edge.
 4. The method of claim 3, whereinsaid band-pass filtering forms a sinusoid of the form H cos(ωn) having afrequency approximating said edge.
 5. The method of claim 4, whereinsaid edge enhancement signal is formed by forming a signal of the form,(L*((cos)²(ωn))+N*(cos)⁴(ωn))+Q*((cos)⁶(ωn)).
 6. The method of claim 5,wherein said signal of the form(L*((cos)²(ωn))+N*(cos)⁴(ωn))+Q*((cos)⁶(107 n) is formed from a signalhaving the form,L*((H cos)²(ωn)/H ²)+N*((H cos)²(ωn)/H ²)² +Q*((H cos)²(ωn)/H ²)³). 7.The method of claim 2, further comprising repeating said analysing andsaid generating for each pixel within said pixel stream, using madjacent pixels in said pixel stream.
 8. The method of claim 7, furthercomprising scaling said pixel stream by a ratio of M:N and furthercomprising scaling said band-pass filtered analysis signal by a ratio ofM:N.
 9. The method of claim 8, wherein said scaling said pixel stream isperformed with a scaler having more taps than a scaler used to scalesaid band-pass filtered analysis signal.
 10. The method of claim 1,wherein said edge enhancement signal contains the third and fifthharmonics of said sinusoid.
 11. The method of claim 10, wherein saidedge enhancement signal further contains the seventh harmonic of saidsinusoid.
 12. The method of claim 11, wherein said edge enhancementsignal further contains the ninth harmonic of said sinusoid.
 13. Themethod of claim 1, wherein said scaled output is generated using aweighted sum of said plurality of adjacent pixels.
 14. The method ofclaim 1, wherein said analysing further comprises determining a measureof high frequency content in said plurality of adjacent pixels, andcontrolling an amplitude of said edge enhancement signal in dependenceon said measure of high frequency content.
 15. The method of claim 14,wherein said determining a measure of high frequency content compriseshigh pass filtering said plurality of adjacent pixels.
 16. The method ofclaim 1, wherein said plurality of adjacent pixels comprises sixteenadjacent pixels.
 17. The method of claim 1, wherein said pixel streamrepresents adjacent pixels in a line of said video image.
 18. The methodof claim 1, wherein said pixel stream represents adjacent pixels in acolumn of said video image.
 19. A method comprising, storing valuesrepresentative of a plurality of adjacent pixels in an image into abuffer; analysing said plurality of adjacent pixels of a video image todetermine a sinusoid representation of an edge in said plurality ofadjacent pixels; generating an edge enhancement signal based on saidanalyzing, said edge enhancement signal including a sum of odd harmonicsof said sinusoid; combining a representation of at least some of saidvalues and said edge enhancement signal, to form an edge enhancedsignal, said analyzing, generating, and combining performed by one ormore circuits.
 20. The method of claim 19, wherein said analysingresults in an analysis signal, and further comprising, combining atleast some of said values of said plurality of adjacent pixels to formsaid representation of said at least some of said values; scaling saidanalysis signal to form said edge enhancement signal.
 21. A methodcomprising, buffering a plurality of values corresponding to a pluralityof adjacent pixels of an image; first combining at least some of saidvalues of said plurality of values to form a scaled representation ofsaid plurality of adjacent pixels; generating an edge enhancement signalbased on using odd harmonics of a frequency content of said plurality ofadjacent pixels; and second combining said scaled representation of saidplurality of adjacent pixels and said edge enhancement signal, whereinsaid buffering, first combining, generating, and second combining isperformed in one or more circuits of an integrated circuit to form anedge enhanced, scaled signal.
 22. The method of claim 21, wherein saidcombining comprises computing a weighted sum of said plurality ofvalues.
 23. A video processor comprising, a buffer for storing aplurality of values wherein each of said values corresponds to each of aplurality of adjacent pixels; a filter for determining a sinusoidrepresentation of an edge in said plurality of adjacent pixels; a signalgenerator for generating an edge enhancement signal including a sum ofodd harmonics of said sinusoid.
 24. The edge enhancement circuit ofclaim 23, wherein said filter comprises a band-pass FIR filter of thetype [ . . . a, b, c, b, a . . . ].
 25. The edge enhancement circuit ofclaim 24, wherein said filter further comprises a band-pass FIR filterof the type [ . . . d, e, 0, −e, −d . . . ].
 26. The edge enhancementcircuit of claim 24, further comprising a second filter for determininga measure of high frequency in said plurality of adjacent pixels. 27.The edge enhancement circuit of claim 23, further comprising a scalerfor scaling an output of said filter.
 28. The edge enhancement circuitof claim 23, wherein said signal generator produces a signal of theform,G*sin(ωn)*(K+(L((cos)²(ωn))+N*(cos)⁴(ωn)+Q*((cos)⁶((ωn))), where K, L, Nand Q are stored constants, G is a gain factor, and ω is representativeof the frequency of said edge in said plurality of pixels.
 29. A methodcomprising, buffering values representative of a plurality of adjacentpixels in an image; generating an edge enhancement signal using a firstsubset of said values; forming a scaled representation of said pluralityof adjacent pixels by combining a second subset of said values; andcombining said scaled representation and a representation of said edgeenhancement signal, to form an edge enhanced, scaled signal, saidbuffering, generating, forming, and combining performed by one or morecircuits.
 30. The method of claim 29, further comprising band-passfiltering said values to form a band-pass filtered analysis signal, andscaling said band-pass filtered analysis signal to form said edgeenhancement signal.
 31. The method of claim 30, wherein said forming ascaled representation of said plurality of adjacent pixels is formedusing a scaler having more taps than a scaler used to form said edgeenhancement signal.
 32. The method of claim 29, wherein said edgeenhancement signal is formed from said band-pass filtered analysissignal using at least one bilinear scaler.